1. Field of the Invention
This invention relates to the field of boiler systems, and more particularly to devices and methods for providing substantially continuous measurement of water conductivity in a boiler.
2. Prior Art
In order to control the rate of build up of scale and other adverse effects in a boiler, it is a common practice to intermittently blowdown a boiler, that is, remove a portion of water therein and replace it with make-up water of lower mineral content and higher purity. The frequency with which this process must be carried out, however, will vary depending upon such factors as the boiler itself and its operating characteristics, and the purity and mineral content of the make-up water. Since the concentration of minerals, salts and other materials in the boiler water affect the electrical conductivity of the water, systems have been developed and are known in the prior art for providing blowdown valve control signals responsive in some form to the apparent conductitivy of the boiler water.
One such prior art system is disclosed in U.S. Pat. No. 3,680,531 entitled "Automatic Boiler Blowdown Control." This patent discloses a specific location for the conductivity probe outside of the boiler region, chosen so as to minimize the entrained bubbles in the water flow around the probe. The patent also discloses electronic circuitry to provide control signals and alarm signals responsive to the apparent conductivity of the water as sensed by the probe. In particular, in this system the probe is located in the blowdown line, and approximately every 15 minutes the blowdown valve is opened and an alternating signal is applied to the probe to provide the conductivity measurement. The blowdown valve is maintained in the open condition for so long as the apparent conductivity remains above a preset control valve, and is closed when the conductivity falls to the control point. Accordingly, if the apparent conductivity is below the control limit, the blowdown valve will close immediately after the conductivity reading is taken, whereas if the conductivity is above the control limit, the blowdown valve will remain open until the apparent conductivity is brought within the control value by the makeup water simultaneously being added to the boiler.
While the prior art systems attempt to minimize the presence of bubbles and flashing in the region of the probe, such adverse affects cannot be entirely eliminated. Accordingly, these systems have attempted to integrate the basic signal from the probe to provide an integrated or average signal in an attempt to minimize the erratic indications caused by the bubbles. However, the presence of bubbles and flashing in the region of the probe only reduces the apparent conductivity of the water. Therefore, the control signal derived by such systems is not responsive only to the conductivity of water, but instead is responsive to the conductivity of the boiler water as reduced by the frequency of occurence and the size of steam bubbles in the water.